Fatty acids arc acyl lipids that are found in a variety of plant tissues, including the triacylglycerols in oil bodies of seeds and fruits, as well as the glycolipids and phospholipids in leaves, roots or shoots. Fatty acids include saturated and unsaturated monocarboxylic acids with unbranched even-numbered carbon chains, such as the unsaturated fatty acids: oleic (18:1, i.e. a C18 chain with a double bond in position 1), linoleic (18:2) and linolenic (18:3) acid.
Significant efforts have been made to manipulate the fatty acid profile of plants, particularly oil-seed varieties such as canola that are used for the large-scale production of commercial fats and oils (see for example U.S. Pat. No. 5,625,130 issued to Grant et al. 29 Apr. 1997; U.S. Pat. No. 5,668,299 issued to DeBonte et al. 16 Sep. 1997; U.S. Pat. No. 5,767,338 issued to Fan 16 Jun. 1998; U.S. Pat. No. 5,777,201 issued to Poutre et al. 7 Jul. 1998; U.S. Pat. No. 5,840,946 issued to Wong et al. 24 Nov. 1998; and U.S. Pat. No. 5,850,026 issued to DeBonte et al. 15 Dec. 1998).
A reduction in the linolenic acid content of plant oils may be desirable for some applications. Low linolenic acid cultivars of B. napus have for example been developed from the cultivar Oro (Röbbelen and Nitsch, 1975, L. Z PflanzenzÜchtg 75:93), by mutagenesis including the low linolenic acid cultivars Stellar (Scarth et al., 1988, Can J Plant Sci 68:509) and Apollo (Scarth et al., 1994, Can J Plant Sci 75:203). The Apollo line has been used to identify molecular markers associated with low linolenic acid loci in a double haploid population derived from a cross between the Apollo line (low linolenic) and a high linolenic line (YN90-1016), using random amplification of polymorphic DNAs and bulk segregant analysis (Somers et al., 1998, Theoretical and Applied Genetics 96(6/7):897). The rapeseed fad3 gene, one of 13 markers identified by Somers et al., supra, was mapped near the locus controlling 14% of the variation in linolenic acid content, confirming a link between the fad3 gene and a low linolenic acid phenotype (Jourdren et al., 1996, Theoretical and Applied Genetics 93:512).
The product of the Fad3 gene is a fatty acid desaturase known variously as delta-15 fatty acid desaturase, linoleic acid desaturase, omega-3 fatty acid desaturase, Fad3 or 15-DES (Arondel et al., 1992, Science 258:1353; Yadav et al., 1993, Plant Physiol. 103:467; WO 93/11245; and WO 98/56239 published 17 Dec. 1998), hereinafter called Fad3. Fad 3 is involved in the enzymatic conversion of linoleic acid to alpha-linolenic acid. In WO 98/56239, DeBonte et al. disclose mutant Fad3 genes, and identify regions of the Fad3 enzyme that are said to contain conserved amino acid motifs which may be mutated to alter fatty acid metabolism in a plant (see Tables 5 and 6 therein). The genomic regions identified by DeBonte et al. generally coincide with the first two of three ‘Histidine Box’ motifs that have been imputed to have a role in the functional activity of the Fad3 enzyme.